The purpose of this project is to determine the precision and accuracy of the video head impulse test (vHIT), a new clinical test of semicircular canal (SCC) function. The vestibular (inner ear balance) system is comprised of two types of sensory organs (semicircular canals and otolith organs) with unique contributions to balance. Loss of vestibular function can occur in one or both labyrinths, in one or both branches of the vestibular nerve, and in one or more vestibular sensory organs. Symptoms of vestibular loss include postural instability, visual blurring, and subjective complaints of dizziness and/or imbalance. Moreover, the incidence of falls is greater in individuals with unilateral or bilateral vestibular loss than in healthy individuals of the same age living in the community. The incidence of dizziness and imbalance increases in populations relevant to VA healthcare: older adults, individuals with traumatic brain injury, and individuals with associated psychiatric disorders. The two most widely used clinical laboratory tests of vestibular function include the caloric test and the rotary chair test. The main advantage of the caloric test is the ability to lateralize a vestibular deficit by stimulating each ear independentl and it is generally considered the 'gold standard' test of horizontal SCC function, especially for the identification of unilateral vestibular losses (e.g., Herdman et al., 1998). The caloric test, however, has a relatively long test time (~30 minutes), is limited to the assessment of the horizontal SCCs, and generates an extremely low-velocity, low-frequency stimulus which is well below the optimal operating range of the sensory receptors of the SCCs. The rotary chair test is considered the 'gold standard' test of horizontal SCC function in patients suspected of having bilateral loss of vestibular function as the caloric test can yield false-positive results. Similarto the caloric test, the rotary chair test is limited to the assessment of horizontal SCC function. Shortcomings include the high cost of the equipment, the space requirements, and the failure to lateralize a unilateral vestibular loss since both labyrinths are stimulated simultaneously. The vHIT is a new clinical test of dynamic SCC function that uses a high-speed digital video camera to record head and eye movement during and immediately after brief, high-acceleration, passive head rotations in the horizontal and vertical planes. The vHIT detects and records abnormal eye movements (overt and covert saccades) and provides measures of either the gain of the vestibulo-ocular reflex or the angle of gaze deviation relative to the angle of head rotation. In contrast to the caloric and rotary chair tests, the vHIT evaluates the function of all six SCCs and provides the capability of assessing the vestibular system using a stimulus that is more physiologically relevant and more representative of the head movements that occur during activities of daily living. Other important advantages of the vHIT relative to the caloric and rotay chair tests include lower cost, shorter test time, greater portability, and increased patient comfort. The specific aims are designed to determine the precision of the vHIT using two examiners, two different vHIT devices, and two test sessions (Specific Aim 1), determine the effect of age on the vHIT (Specific Aim 2), establish normal reference intervals (Specific Aim 3), and determine the accuracy (sensitivity and specificity) of the vHIT in identifying patients with unilateral and bilateral vestibular losses (Specific Aim 4). To address Specific Aims 1-3, data wil be obtained from the following 5 groups of Veteran participants: (1) young normal adults (18 to 40 years of age, n = 25), (2) older normal adults (60 to 89 years of age, n = 25), (3) patients wit total unilateral vestibular losses (18 to 89 years of age, n = 15), (4) patients with partial unilateral vestibular losses (18 to 89 years of age, n = 15), and (5) patients with bilateral vestibular losses (18 to 89 years of age, n = 15). To address Specific Aim 4, the results of the index test (vHIT) will be compared with the results of the reference tests for unilateral vestibula losses (caloric test) and bilateral vestibular losses (rotary chair test) in 480 patients (18 to 89 years of age) evaluated in the Vestibular/Balance Clinic at the Mountain Home VAMC.